1. Field of the Invention
The present invention relates to an electric machine, such as a rotating electric machine or a linear motor, as well as to a technique for reducing eddy current loss which occurs in a field pole unit.
2. Description of the Background Art
Conventionally, a rotating electric machine provided with a field pole unit having a plurality of permanent magnets and an armature including concentratedly wound armature coils has been used in various applications. “Concentrated winding” is a coil-winding structure in which coils are concentratedly wound on teeth of an armature. As a result of recent advances in machine-assisted automatic coil-winding techniques, concentrated winding is widely used today in the manufacture of rotating electric machines, chiefly for compact motors including servomotors. A majority of energy loss occurring in such small-sized motors is attributed to copper loss, iron loss and mechanical loss, so that the eddy current loss occurring in the field pole unit does not usually pose a serious problem.
Although large-sized motors whose wattage exceeds a few kilowatts have conventionally been provided with an armature including distributedly wound armature coils, it is increasingly desired to employ concentratedly wound armature coils having smaller coil ends to achieve space savings. In fact, there is a strong need today for size reduction of the coil ends in motors used as an elevator traction machine or a motor for directly driving a stage of a machine tool, for example.
In large-sized motors rated over a few kilowatts, however, eddy current loss occurring in a field pole unit constitutes an appreciable part of a total energy loss. In addition, magnets like rare-earth magnets characterized by high remnant magnetic flux density and high remanence have increasingly been used as field pole magnets in recent years. For example, neodymium-iron-boron (Nd—Fe—B) permanent magnets often used as field pole magnets today tend to produce eddy currents as compared to ferrite permanent magnets, and thus have a problem that an eddy current loss occurring in a field pole unit causes a reduction in motor efficiency and a temperature increase of the field pole unit results in demagnetization of the field pole magnets. Even if the field pole magnets are not demagnetized, the temperature increase of the field pole unit would cause a reduction in the remnant magnetic flux density and a resultant decrease in a total quantity of magnetic flux produced by the field pole magnets. To make up for a loss in motor power caused by this temperature increase of the field pole unit, it is necessary to flow an increased amount of armature current, which causes a problem that the motor efficiency further decreases due to an increase in copper loss.
It might be possible to reduce the eddy current loss by employing a laminated core built up of laminations of steel sheets and distributedly winding armature coils to suppress magnetic field harmonics produced by armature currents. A conventional approach directed to the solution of the aforementioned problems is described in Japanese Patent Application Publication No. 1996-289491. According to the Publication, a core carrying field poles is built up of a plurality of blocks formed by laminating multiple steel sheets, in which the steel sheets are electrically insulated in a laminating direction thereof to prevent eddy current loss occurring in a field pole unit. Another conventional approach is shown in Japanese Patent No. 3280351, in which a core is not a laminated core but built up of a plurality of solid yokes carrying field poles. In this approach, the solid yokes are electrically separated from one another so that a path through which eddy currents will flow is not created.
The prior art to which the invention is directed discloses structures in which the core (field pole yoke) is built up of laminated steel sheets or of electrically separated, or divided, solid yokes to prevent the eddy current loss occurring in the field pole unit as mentioned above. The former approach has a problem that the laminated field pole yoke structure requires an capital investment in manufacturing equipment including a large-sized metal die and press, resulting in high manufacturing cost. The latter approach also has a problem that the divided solid yoke structure requires greater manpower or complex processes due to an increase in the number of constituent components, resulting in high manufacturing cost. Additionally, the divided solid yoke structure could produce uneven magnetic flux densities in a magnetic gap formed between the field pole unit and a armature due to uneven thickness of insulating material inserted between one solid yoke and another, eventually causing acoustic noise or vibration. On the other hand, motors with distributedly wound armature coils have a problem that these motors have large coil ends.